A novel stir bar sorptive extraction (SBSE) procedure termed sequential SBSE was developed. Compared to conventional SBSE, sequential SBSE provides more uniform enrichment over the entire polarity/volatility range for organic pollutants at ultra-trace levels in water. Sequential SBSE consists of a SBSE performed sequentially on a 5-mL sample first without modifier using one stir bar, then on the same sample after addition of 30 % NaCl using a second stir bar. The first extraction with unmodified sample is mainly targeting solutes with high Ko/w (log Ko/w > 4.0), the second extraction with modified sample solution (containing 30 % NaCl) is targeting solutes with low and medium Ko/w (log Ko/w < 4.0). After extraction the two stir bars are placed in a single glass desorption liner and are simultaneously desorbed.
The desorbed compounds were analyzed by thermal desorption and gas chromatography – mass spectrometry (TD – GC – MS). Recovery of model compounds consisting of 80 pesticides (organochlorine, carbamate, organophosphorous, pyrethroid, and others) for sequential SBSE was evaluated as a function of log Ko/w (1.70 - 8.35). The recovery using sequential SBSE was compared with those of conventional SBSE with or without salt addition (30 % NaCl). The sequential approach provided very good recovery in the range of 82 to 113 % for most of the solutes, and recovery less than 80 % for only 5 solutes with low Ko/w (log Ko/w < 2.5), while conventional approaches (with or without salt addition) showed less than 80 % recovery for 23 and 41 solutes, respectively. The method showed good linearity (r2 > 0.9900) and high sensitivity (limit of detection: < 10 ng/L) for most of the model compounds even with the scan mode in the MS. The method was successfully applied to screening of pesticides at ng/L level in river water samples.